The present invention relates to a directional coupler type optical device of a novel construction and a driving method therefor, and more particularly, to a directional coupler type optical device, capable of enjoying a high extinction ratio when used as an optical switch, polarizing splitter, optical modulator, light-wave divisional multiplexer, etc., and a driving method therefor.
Recently, various optical devices having a directional coupler of a waveguide type have been developed, and optical switches, polarizing splitters, optical modulators, light-wave divisional multiplexers, etc. using these devices have been proposed.
FIGS. 1 and 2 show examples of plane patterns of conventional optical devices of a directional coupler type. The device shown in FIG. 1 is a 2-input/2-output device, while the device shown in FIG. 2 is a 1-input/2-output device.
In FIG. 1, a junction C0 of a length L is formed by arranging two optical waveguides A and B of equal widths W close to each other in parallel relation, with a distance G for evanescent coupling between them.
Curved optical waveguides D1, D2, D3 and D4 with a path width W and curvature radius R are optically connected to the respective incidence ends A1 and B1 and emergence ends A2 and B2 of the optical waveguides A and B of the junction C0, respectively, thus forming an incidence-side lead section C1 and an emergence-side lead section C2. Also, straight optical waveguides E1, E2, E3 and E4 with the path width W are optically connected to the curved optical waveguides D1, D2, D3 and D4, respectively, and a distance GF is kept between the respective path-width centers of each corresponding two of the waveguides E1 to E4. Electrodes F1, F2, F3 and F4 are mounted on the optical waveguides A and B of the junction C0. The electrodes F1 to F4 function as propagation constant control means which controls the propagation constant of the optical waveguides situated individually right under the electrodes for a desired value by introducing specific electrical signals from the electrodes.
If the straight optical waveguide E1 is an incidence port, the straight optical waveguides E3 and E4 connected to the emergence-side lead section C2 serve as a through port and a cross port, respectively.
Basically, the 1-input/2-output device of FIG. 2 has the same configuration as the 2-input/2-output device of FIG. 1. In the device of FIG. 2, one straight optical waveguide E0 is optically connected to only the incidence end A1 of the one optical waveguide A in a direct manner, thus forming the incidence-side lead section C1. In FIGS. 1 and 2, like reference numerals are used to designate those elements which are common to the two devices. In the device of FIG. 2, the straight optical waveguide E0 is an incidence port, and the straight optical waveguides E3 and E4 connected to the emergence-side lead section C2 serve as a through port and a cross port, respectively.
In order to incorporate these devices in a fiber communication system, which is going to be practically used, it is necessary to prevent errors attributable to cross talk. Thus, the devices are expected to be subject to less cross talk, that is, to have high extinction ratio characteristics.
In the case of the device shown in FIG. 1, a theoretically perfect cross mode can be established by applying proper electrical signals from the electrodes F1, F2, F3 and F4.
In the case of a through mode, however, slight coupling is produced between the respective curved optical waveguides of each of the incidence- and emergence-side lead sections C1 and C2. In this case, therefore, a perfect through mode cannot be established, and the extinction ratio can be about 25 dB at the highest.
In the case of the device shown in FIG. 2, no coupling is produced corresponding to the one between the optical waveguides of the incidence-side lead section of the device shown in FIG. 1, so that the extinction ratio for the through mode can be about 35 dB, which is higher than that of the device shown in FIG. 1. However, the device of FIG. 2 cannot enjoy the symmetry between the coupling at the incidence-side lead section and that of the emergence-side lead section of the device of FIG. 1, and slight coupling is produced between the optical waveguides of the emergence-side lead section. According to the device of FIG. 2, therefore, the extinction ratio for the cross mode can be only about 20 dB at the highest.
Thus, the conventional devices, which have a low extinction ratio for the through or cross mode, cannot exhibit high extinction ratio characteristics for both the through and cross modes.
Since the extinction ratio characteristic of the optical device is defined by the lower one of the extinction ratios for the through and cross modes, only a low value can be obtained as the extinction ratio of the whole device.
The extinction ratio used here is a value given by 10 log.sub.10 (.vertline.r.vertline..sup.2 /.vertline.s.vertline..sup.2), where .vertline.r.vertline..sup.2 is the output power of the through port, and .vertline.s.vertline..sup.2 is the output power of the cross port.
Among optical devices constructed in this manner, known examples of those which have relatively high extinction ratio characteristics include an optical switch with an extinction ratio of about 27 dB reported in Technical Digest Integrated and Guide-wave Optics '86 by P. Granestrand et al. and a polarizing splitter with an extinction ratio of about 28 dB reported in the 1990 Autumn National Meeting C-216 of the Institute of Electronic Intelligence and Communication Engineers of Japan by H. M. Mak et al.
H. M. Mak et al. also proposed a device shown in FIG. 3 in the 1991 Spring National Meeting C-224 of the Institute of Electronic Intelligence and Communication Engineers of Japan.
Theoretically, this device can obtain an extinction ratio of at least 40 dB or thereabout.
The device shown in FIG. 3 can, however, enjoy this high theoretical extinction ratio only when the dimensional parameters of the individual partial junctions and the like are substantially equal to their theoretical values.
In actually manufacturing the device, however, these individual partial junctions and the like cannot always be formed with a dimensional accuracy based on the theoretical values obtained by calculation, and their dimensional parameters sometimes may delicately deviate from the theoretical values.
In such a case, the actual states of coupling between the optical waveguides at the individual partial junctions depart from theoretical states of coupling obtained by calculation, so that the extinction ratios for the cross and through modes inevitably lower.